Human respiratory syncytial virus (RSV) infects nearly everyone worldwide early in life and is responsible for considerable mortality and morbidity. In the United States alone, RSV is responsible for 75,000-125,000 hospitalizations yearly, and worldwide conservative estimates conclude that RSV is responsible for 64 million pediatric infections and 160,000 pediatric deaths. Another unusual feature of RSV is that severe infection in infancy can be followed by years of airway dysfunction, including a predisposition to airway reactivity. RSV infection exacerbates asthma and may be involved in initiating asthma.
RSV is a member of the Paramyxoviridae family and, as such, is an enveloped virus that replicates in the cytoplasm and matures by budding through the host cell plasma membrane. The genome of RSV is a single, negative-sense strand of RNA of 15.2 kilobases that is transcribed by the viral polymerase into 10 mRNAs by a sequential stop-start mechanism that initiates at a single viral promoter at the 3′ end of the genome. Each mRNA encodes a single major protein, with the exception of the M2 mRNA, which has two overlapping open reading frames that encode two separate proteins. The 11 RSV proteins are: the RNA-binding nucleocapsid protein (N), the phosphoprotein (P), the large polymerase protein (L), the attachment glycoprotein (G), the fusion protein (F), the small hydrophobic (SH) surface glycoprotein, the internal matrix protein (M), the two nonstructural proteins NS1 and NS2, and the M2-1 and M2-2 proteins encoded by the M2 mRNA. The RSV gene order is: 3′-NS1-NS2-N-P-M-SH-G-F-M2-L. Each gene is flanked by short transcription signals called the gene-start (GS) signal, present on the upstream end of the gene and involved in initiating transcription of the respective gene, and the gene-end (GE) signal, present at the downstream end of the gene and involved in directing synthesis of a polyA tail followed by release of the mRNA.
The development of live-attenuated vaccines has been in progress since the 1960's but has been complicated by a number of factors. For example, RSV grows only to moderate titers in cell culture and is often present in long filaments that are difficult to purify and can readily lose infectivity during handling. Another problem is that the magnitude of the protective immune response is roughly proportional to the extent of virus replication (and antigen production). Thus, attenuation is accompanied by a reduction in immunogenicity, and it is essential to identify a level of replication that is well tolerated yet satisfactorily immunogenic. These studies can only be done in humans, because RSV does not replicate efficiently in most experimental animals, such as rodents and monkeys. Chimpanzees are more permissive but usually are not readily available. Another obstacle is the difficulty in developing attenuating mutations. Another obstacle is genetic instability that is characteristic of RNA viruses, whereby attenuating mutations can revert to the wild-type assignment or to an alternative assignment that confers a non-attenuated phenotype.
Until recently, RSV vaccine candidates were developed by conventional biological means. These previous biologically-derived candidates were either under-attenuated or over-attenuated, and genetic instability was observed in some cases.